Synthesis of alkaline earth metal halophosphate phosphor activated by antimony and manganese requires that the activators be provided in their lowest oxidation states, that is, trivalent (3.sup.+) antimony and divalent (2.sup.+) manganese. However, during firing the antimony and manganese tend to oxidize to form pentavalent (5.sup.+) antimony and tetravalent (4.sup.+) manganese. Pentavalent antimony and tetravalent manganese are not incorporated into the phosphor lattice and are thus wasted. The presence of antimony which is not in the trivalent state during the synthesis of alkaline earth metal halophosphate phosphor activated by antimony and manganese has been associated with reduced luminescence efficiency.
The desirability of maintaining a high concentration of trivalent antimony content during synthesis of alkaline earth metal halophosphate phosphor has been recognized. U.S. Pat. No. 3,468,812 to Wanmaker et al. discloses the use of antimonite, (SbO.sub.2).sub.n.sup.n-, instead of the more volatile antimony trioxide, Sb.sub.2 O.sub.3, as the source of antimony in order to retain more trivalent antimony during synthesis of alkaline earth metal halophosphate phosphor. U.S. Pat. No. 3,378,499 to Hickok discloses that greater concentrations of trivalent antimony may be obtained in calcium halophosphate phosphor if calcium chlorospodiosite is used as the chloride source instead of ammonium chloride. British Patent 1,371,968 to Philips discloses the use of two firing steps to ensure the availability of sufficient amounts of trivalent antimony in the resulting phosphor.
While such methods do increase the amount of trivalent antimony during synthesis of the phosphor, they do not ensure that all of the antimony and manganese which are present during synthesis are available in the trivalent and divalent forms, respectively. Optimum performance of the phosphor cannot be obtained unless the activators are present in their lowest oxidation states.
U.S. Pat. No. 3,430,089 to Sarver et al. discloses the addition of divalent zinc to calcium halophosphate phosphor to improve light output and maintenance.
We have determined that optimum improvement in light output and maintenance is obtained when the divalent zinc compound is zinc phosphide. Additionally, greater amounts of input zinc are retained when zinc phosphide is used as the source of zinc. The use of other divalent zinc compounds, including zinc oxide, does not provide the benefits previously associated with the use of divalent zinc generally.
It would be an advantage in the art to provide a method of making an improved alkaline earth metal halophosphate phosphor activated by antimony and manganese.
It would also be an advantage in the art to provide a method of making alkaline earth metal halophosphate phosphor activated by antimony and manganese in which the activators are present during synthesis in their lowest oxidation states.